The present invention relates to a method and a device for liquefying a fluid or a gaseous mixture formed at least in part of a hydrocarbon mixture, for example a natural gas.
Natural gas is currently produced at sites remote from the utilization sites and it is commonly liquefied before being transported over long distances by LNG tanker or stored in liquid form.
Throughout the specification, "natural gas" shall be understood to be a mixture whose major component is methane but which may also contain other hydrocarbons and nitrogen in whatever state (gaseous, liquid, or diphasic). At the outset, natural gas is mainly in the gaseous state and at a pressure such that, during the liquefaction stage, it may be in different coexisting states, for example liquid and gas, at any given moment in time.
Natural gas can be liquefied by three main methods known in the prior art and summarized hereinbelow.
A first method consists of operating by means of three cooling cycles in series, each of which operates with a pure substance as a coolant. A first cycle operating with propane condenses ethylene under pressure to a temperature of approximately -35.degree. C. By vaporizing the ethylene at a pressure close to atmospheric pressure in a second cycle, methane is condensed under pressure to a temperature of approximately -100.degree. C. By vaporization of methane, the LNG produced is sub-cooled and can thus be expanded for storage and transportation at a pressure close to atmospheric pressure.
A second method frequently used is described in the prior art, particularly in U.S. Pat. Nos. 3,735,600and 3,433,026, consists of replacing the latter two cycles, an ethylene cycle and a methane cycle, by a single cycle operating with a mixture of coolants. The operating principle of such a cycle is shown schematically in FIG. 1.
The coolant mixture is compressed in a compressor K1, and is then cooled by the ambient cooling fluid available, water or air, in exchanger C1 which it leaves through pipe 1. It is then sent to cooling stage (I) in which it is cooled by means of a propane cycle. At the outlet from stage (I) the mixture leaves through pipe 2 in the liquid-vapor state. The two phases thus obtained are separated in separator B1. The liquid fraction is sent to cooling stage (II) in which it is sub-cooled then expanded through expansion valve VI. As it vaporizes, it cools the natural gas which arrives in cooling stage (II) through pipe 11' as well as the fraction of vapor coolant coming from separator B1, which is sent to cooling stage (II) through pipe 4, down to approximately -100.degree. C. so that the natural gas and the vapor fraction of the coolant mixture can be condensed.
The condensed fraction of coolant mixture thus obtained is sub-cooled then expanded through expansion valve V2. As it vaporizes, it sub-cools the natural gas down to a temperature of approximately -160.degree. C. The liquefied natural gas under pressure that leaves cooling stage (II) through pipe 14 is expanded through expansion valve V3 to a pressure close to atmospheric pressure, producing the LNG which is evacuated through pipe 15.
The natural gas enters cooling stage (I) through pipe 10 and leaves it through pipe 11. It is then sent, as shown in dotted lines in the figure, to a fractionation device from which it is sent by a pipe 11' to cooling stage (II).
A third possibility consists of operating with a single cycle employing a single compressor K1 using the arrangement shown schematically in FIG. 2. The coolant mixture leaving compressor K1 is partially condensed in exchanger C1. The two phases, liquid and vapor, thus obtained are separated in separator B2. The vapor fraction evacuated through pipe 1 plays the same role as the coolant mixture which, in the case of the arrangement shown in FIG. 1, is sent to cooling stage (II). Cooling stage (II) operates similarly in the two arrangements shown schematically in FIGS. 1 and 2. The liquid fraction evacuated from separator B2 through pipe 5 is sub-cooled in cooling stage (I) then expanded through expansion valve V4. Its vaporization furnishes the cooling required in cooling stage (I).
French patent FR 95/15623 by the applicant also proposes operating under selected pressure and temperature conditions to obtain, at the outlet of cooling stage (I), a condensed entirely single-phase coolant mixture in the liquid phase or in dense phase.
"Dense phase" shall be understood hereinbelow to mean a phase at a pressure greater than the cricodenbar pressure of the mixture and at a pressure and temperature such that, by isentropic expansion, it can form a saturated liquid phase.
In this case one can operate according to the diagram in FIG. 3. In this example, cooling stage (II) is comprised of two separate heat exchange areas E1 and E2. The condensed coolant mixture arriving via pipe 2 in cooling area (II) is first vaporized at an intermediate pressure in heat exchange area E1 then vaporized at low pressure in heat exchange area E2. The natural gas leaving heat exchange area E1 through pipe 12 is thus cooled in heat exchange area E1 to a temperature between -100 and -120.degree. C. for example then cooled in heat exchanger area E2 to a temperature of approximately -160.degree. C. The outlet temperature of cooling stage (I) can in this case be for example between -60 and -80.degree. C. while it is approximately -30 to -35.degree. C. when cooling stage (I) is cooled by propane.
The natural gas can then be collected by pipe 11 at an intermediate point of cooling stage (I) and sent to a fractionation device, and re-injected by pipe 11' into exchange area (I).
To reach the cooling temperatures required at the outlets of exchange areas E1 and E2, a coolant mixture comprising methane, ethane, and nitrogen is used. Because of the relatively narrow vaporization temperature range, between 30 and 50.degree. C. for example, it is however necessary to use a coolant mixture with a very high methane concentration which does not allow a regular enthalpy-temperature profile to be obtained, bringing about temperature differences in heat exchange areas E1 and E2 which can be locally considerable. The result is a deterioration in performance.